Polyolefin resins, such as polyethylene, for use in nonwovens are difficult to produce and pose additional challenges compared to polyolefin resins suitable for films. This is because the material and processing requirements for production of fibers are much more stringent than for producing films. For the production of fibers, the residence time available for structure formation is typically much shorter and flow characteristics are more demanding on the material's physical and rheological properties. Also, the local shear/extensional rate and shear rate are much greater in fiber production than other processes and, for spinning very fine fibers, small defects, slight inconsistencies, or phase incompatibilities in the melt are not acceptable for a commercially viable process.
Generally, smaller fiber diameters will enable softer nonwovens. Softer nonwovens are desirable as they are gentler to the skin, feel right to the touch and help provide a more garment-like aesthetic for diapers, wipes and similar products.
Another desirable attribute sought in the fibers comprising nonwovens, besides softness, is abrasion resistance. Abrasion resistance is also important since it ensures that both the fibers and the nonwovens possess sufficient mechanical integrity during use so as to not fall apart and produce undesirable fuzz or lose aesthetics.
Nonwovens that are capable of high extensibility at relatively low force are also desired. These can be used to provide sustained fit in products, such as diapers and the like, and facilitate the use of various mechanical post-treatments. Typically, it has been found that having both a smaller fiber diameter and an easy to extend fiber are difficult to achieve. This is because, when the fiber diameter is reduced, it is commonly done by increasing the spinning speed or draw ratio during spinning which decreases extensibility of the fiber in post mechanical treatment due to increased polymer orientation.
More recently, there has also been a growing need in the industry for nonwovens that can also exhibit significant extensibility or cold-drawability when used in disposable products. Indeed, for absorbent articles such as diapers and catamenials, solid-state activation processes have become an integral part of the fabrication of many chassis components. Such processes can provide important functional benefits: improved softness or hand that increase a nonwoven's comfort and feel; added loft, texture or aperturing that enhance visual appearance, alter transport properties or desirably modify mechanical properties. In such processes, however, the nonwoven needs remain intact after being stretched at high a strain rate and maintain all or most of its original mechanical integrity.
One way the art has used to address these issues is to blend various polymeric resins. For example, U.S. Pat. Nos. 4,842,922 and 4,990,204 discuss blends of a high molecular weight linear polyethylene and a low molecular weight linear polyethylene for use in production of spun-bonded fibers. U.S. Pat. No. 5,122,686 discusses blends of high and low molecular weight linear polyethylenes for extrusion of staple fibers. Also, U.S. Pat. No. 6,015,617 describes blends of a first homogeneous ethylene/α-olefin interpolymer and a second polymer (either an ethylene homopolymer or an ethylene/α-olefin interpolymer) directed to improved bond performance. However, there is a continuing need for improved nonwoven materials.
As noted above, there clearly exists an unmet need for highly extensible nonwovens with fibers that can be made from commercially available thermoplastic resins without the need for high cost specialty polymers or elastic polymers or complex manufacturing processes. It is well known that, as spinning speeds increase, molecular orientation increases, stress to further deform the fiber increases and fiber elongation decreases. This is ideal for producing low denier fibers with high strength and low deformability. However, producing fine fibers with high extensibility at an affordable cost remains a very significant challenge.
There is also a need for a polyethylene material suitable for use in a fibrous nonwoven which is readily extensible and has improved abrasion resistance.